Reflectance-based instruments have long been in use in a variety of applications. One type of reflectance-based system is referred to as a “reflectometer”, used to perform tests in certain medical and laboratory applications. In a typical form, a reflectometer includes one or more light sources configured to generate one or more light signals at given wavelengths. An object under test receives the signal and reflects a portion thereof—referred to as “reflectance”. Reflectance is typically considered to be unit-less because it is defined as the ratio of the light actually leaving a sample to the amount that would leave if none were absorbed. In recent years, the National Institute of Standards and Technology (NIST) has defined reflectance in terms of this kind of mathematical model, rather than provide a physical reflectance standard. One or more detectors or sensors are oriented to receive the reflected signals. A processor analyzes the characteristics of the received reflected signals and produces a test result.
Such reflectometers are sometimes used for performing tests on a reagent test strip. In such a case, the test pads on the test strip may be incrementally tested to determine the presence of analytes in a liquid test sample absorbed into the test pads. Such systems may be used for performing urinalysis tests, as one example. That is, the level or presence of an analyte in a liquid test sample can be determined by causing a given test pad to absorb some of the liquid test sample, (e.g., a sample of urine) and then by reading associated reflectance values for the test pad with a reflectometer. Based on the reflectance characteristics of the signal reflected by the test pad, the reflectometer determines the presence or level of the analyte in a given test pad. As an example, a test pad changes color to indicate the level or presence of the analyte in response to absorption of urine from a urine sample. The characteristics of a reflected signal are a function of the make-up and color of the test pad and the wavelength of the light source. Consequently, a change in color of a test pad causes a corresponding change in the characteristics of the reflected signal.
Test strips are typically produced according to industry accepted formats. In the case of urinalysis reflectometers, test strips can come in formats having different lengths, such as, for example, 3.25 inch lengths or 4.25 inch lengths. Within each format, a test strip is defined according to its configuration, i.e., the number, types and order of test pads included on the test strip. Generally, each test strip configuration is uniquely identified. Implicit in a test strip identification and/or confirmation, therefore, is the test strip format and the test pad configuration. As will be appreciated by those skilled in the art, such test pads may include, for example, pH, ketone, nitrite, and glucose test pads. In order for the reflectometer to produce valid results, the test strip must be identified by format and configuration, so that the reflectometer has a proper context to evaluate the received reflected signals, or reflectance values derived therefrom. That is, a reflectometer needs to know that a received reflected signal is produced by, for example, a glucose test pad or a ketone test pad.
Reagent cassettes can also be tested using a reflectometer, in a manner very similar to that used for the test strip. Such reagent cassettes include a test region that provides visual indications of test results, similar to the test pads of the test strips. The test region can produce a series of lines that embody the test results.
There are numerous rapid test assays in the market utilizing immunochromatography devices. Most are limited to YES/NO answers because of their poor quantitation (i.e., poor ability to measure or estimate quantity with precision). To achieve a higher level of quantitation, reflectometers can be used to subjectively examine the colored bands formed on a test product. However, reflectance measurements in a reflectometer are prone to many sources of error because the positioning and height of the test strip or reagent cassette can greatly alter the amount of photons that reach the detector. Even slight differences in the height of a test product can alter the reflectance value obtained, thus becoming a source of error when measuring analyte concentration by reflectance measurements.
Some systems attempt to address these circumstances with a straightforward ratio-ing of wavelengths. The problem with just rationing the wavelengths is that there is great difficulty in associating meaning to such numbers and they do not lend themselves to processes or algorithms that utilize the related reflectance measurements for generating subsequent information or test results. For example, one process that uses reflectance values is the “K/S” transformation for linearizing reflectance measurements, which is given by the equation:K/S(R)=(1−R)2/(2*R)  (1)Here, a ratio of wavelengths would not result in a reflectance value R useful in such an equation. Similar problems would be realized in other functions that rely on the use of R.